Step 1Remove and discard the rind from the watermelon and cut the flesh into 1-inch cubes to make 3 cups (about 20 ounces), saving the trimmings. Place the trimmings (20 to 25 ounces) in a blender jar and blend to a pulpy mixture. You should have 2 to 2½ cups. Add any juice from the chopping board to the pulp.
Step 2In a wide sauté pan, heat the oil over medium-high. Add the cumin seeds and sauté until browned and very fragrant, about 45 seconds. Add the garlic and ginger. Cook until golden, about 1 minute, but do not brown, then add the watermelon pulp, coriander seeds, chili powder and turmeric and stir to combine. Reduce the heat to medium-low and simmer until the sauce is reduced by half and the color has deepened to a rust-orange tone, about 15 minutes.
Step 3Add the watermelon cubes, serrano chile, lemon juice and salt, to taste, and increase heat to medium. When the watermelon is heated through, 4 to 5 minutes, add the chopped cilantro. Serve hot with basmati rice tossed with cooked green peas, quinoa or fluffy couscous.